Monochrome Spectroscope to Examine Planck Radiation Curves

A spectroscope can be used to examine and analyze light sources. It splits the light into its individual wavelengths. This requires a glass prism or a diffraction grating. A camera module/webcam then records the spectrum.

However, if you want to examine the solar spectrum, color cameras are not suitable. These cameras have a so-called Bayer mask consisting of color filters in front of their color sensor. These absorb part of the incident light and thus distort the light spectrum under investigation.

For example, the solar spectrum should correspond to a Planck radiation curve with a maximum at around 500 nm. However, if recorded with a color camera, the intensity curve does not follow the Planck radiation curve due to the Bayer filter. In the wavelength range around 550 nm, the spectrum has a significantly lower intensity than it should.

A black-and-white camera without a Bayer filter can remedy this problem. This does not distort the light spectrum under investigation. If you record the solar spectrum with such a black and white camera, you will receive a beautiful Planck radiation curve as desired.

This spectroscope requires only a few things:

1. a black-and-white camera module (Amazon)
2. a diffraction grating with 500 or 1000 lines/mm
3. two razor blades
4. a plastic case (f.e. link)
5. the Theremino spectroscope software: link1, link2

To mount the camera module in the plastic housing, I used a small piece of wood. I attached another piece of wood above the lens and glued a magnet to it. I then used this magnet to fix the diffraction grating in front of the lens. Additional magnets can be used to adjust the ideal position of the diffraction grating. I created the slit using two razor blades. I then fixed them at a distance of approximately 50 µm, exactly above the entrance hole. To prevent stray light, I also mounted black foam rubber on the long side of the plastic housing.

Now connect the camera's USB cable to the computer and launch the Theremino spectroscope software. You can then calibrate the spectrum using an older energy-saving light bulb. Energy-saving light bulbs have a multitude of spectral lines. Using the mouse, you then move the spectrum in the Theremino software until the peaks are at the correct wavelength.

The spectral range I recorded with the spectroscope ranges from approximately 200 nm to 1300 nm, which is ideal for Planck radiation curves.

A halogen lamp is a very good example of a blackbody radiator. The spectrum depends on the temperature of the filament. The hotter it is, the higher the radiation intensity and the further the spectrum shifts towards blue. This is stated by Wien's displacement law. The temperature of the filament can be easily determined using the electrical resistance R. To determine the temperature T, you only need the resistance at room temperature R_20 and then the resistance R during operation. From R_20 and R, the temperature T can then be calculated (see appendix).

The Planck radiation curves are clearly visible, as is their shift towards shorter wavelengths with increasing temperature. The only flaw: The camera module can only capture wavelengths up to approximately 1000 nm. However, parts of the Planck radiation curve lie at longer wavelengths. Therefore, the recorded radiation curves do not fully correspond to the theory.

Finally, we record the solar spectrum with our spectroscope. This corresponds to a Planck radiation curve at around 5500 K with a maximum at approximately 500 nm. Not only the beautiful radiation curve is visible, but also individual absorption lines, the so-called Fraunhofer lines. These consist of the individual elements of the sun, including the hydrogen lines of the Balmer series (H-alpha, H-beta, etc.).

If you're interested in other exciting physics projects, you can find my homepage and my YouTube channel here:

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In that spirit, stay curious and Eureka!